Many silicate compositions can easily be melted and formed as a glass. However, only certain compositions have a viscosity which is suitable at reasonable temperatures for efficiently forming mineral wools. For example, a viscosity of approximately 10 poise (log n=1.0) at 2600.degree. F. is recommended in the literature as an "optimum" high temperature viscosity for forming mineral wool fibers. However, many compositions which have viscosities in this range also have liquidus temperatures which are too high, being near or above the fiber forming temperature, which can lead to devitrification during fiber formation.
Desired uses of mineral wools are as high temperature insulation or as a constituent in fire resistant ceiling tile. These tiles are low density composities consisting primarily of mineral wool, clay, organic binders and organic fibers. During a fire, a number of complex reactions take place in the composite which include organic binder and fiber burn-out, low temperature bonding between inorganics and high temperature deformation and crystallization of the mineral wool. Therefore, mineral wools which are to be used in these high temperature applications should have a high softening temperature and should crystallize sufficiently rapidly so as to be "fire resistant." This crystallization will prevent the fibers from softening to the point where the ceiling tiles or boards will fall out of their suspension system during a fire. This must be considered in the formulation of the mineral wool since many compositional modifications which can be made to obtain the desired viscosity and liquidus will also affect the rate of crystallization and the softening point of the fibers.